Pin chip recovery

ABSTRACT

A method is described for separating pin chips from a wood furnish of wood chips, pin chips and fines mixture. In the method, the wood chips are separated from the pin chip and fines mixture and treated with a surfactant to lower the interfacial surface tension between the pin chip and the fibers. The resulting treated pin chip and fines mixture is then mechanically separated into the individual pin chip and fines fraction.

FIELD

This application relates to the recovery of wood pin chips from a mixture of wood chips, pin chips and fines.

DESCRIPTION

In the wood pulping industry, trees are conventionally classified as either hardwood or softwood. In the practice of the present application, wood chips are derived from softwood tree species such as, but not limited to: fir (Balsam fir), Douglas fir, pine (Eastern white pine and Loblolly pine), spruce (White spruce), larch (Eastern larch), cedar, and hemlock (Eastern and Western hemlock). Examples of hardwood species from which wood chips are derived include, but are not limited to: acacia, alder (Red alder and European black alder) aspen (Quaking aspen), beech, birch, oak (White oak), gum trees (eucalyptus and Sweetgum), poplar (Balsam poplar, Eastern cottonwood, Black cottonwood and Yellow poplar), gmelina and maple (Sugar maple, Red maple, Silver maple and Bigleaf maple).

Wood chips can come from chipping whole logs or from saw logs. Those wood chips used in the pulp and paper industry are produced mainly by means of disc chippers. Chip quality depends on different factors including the diameter of the logs, the amount of wood to be chipped and the dry solids content of the wood; these vary constantly. Depending on the type of chipper and various wood factors, considerable variation can result in the various fractions of chips that are produced.

One widely used method for classifying the wood chips is by SCAN-CM 40: 94. In the method, oversized chips are retained on a 45 mm hole screen, over-thick chips on a 8 mm slot screen, accepts on 7 mm hole screen, pin chips on a 3 mm hole screen and fines are collected in a pan. Accepts, pin chips and fines are defined in this application by this classification.

Screening wood chips frees the chips from small and large fractions into smaller chip size fractions that could have a detrimental effect on the pulping process. The large-size fractions are usually treated by means of a rechipper or crushed by means of a roller into a more suitable size for the process. Accepts are used in the pulping process. The pin chip fraction constitutes a good raw material in terms of fiber but, due to surface tension properties, have fines associated with them. As defined herein, fines are that fraction of a wood furnish that passes through a 3 mm screen in the SCAN-CM 40: 94 method. This latter fraction can cause problems in the pulping process by causing digester blockage and must therefore be pulped in special digesters designed to handle smaller particle sizes. Suitable digesters include the M&D or the Pandia digesters. These digesters are designed to have significantly lower residence time, higher liquor-to-wood ratios and higher cooking temperatures than conventional digesters.

In the present application, a wood furnish comprised of wood chips, pin chips and fines is first mechanically separated to remove the oversized and acceptable wood chips. The residual pin chip and fines mixture that passes through a 3 mm gyratory screen is then treated with a surfactant which releases a portion of the fines fraction from the pin chips and the treated mixture classified to determine the pin chips and fines fraction. In practice, the pin chips can be collected and fed to the digester at a controlled rate with the acceptable chips or they are recirculated with the acceptable chip and used in the digester. The fines are usually used for hog fuel but could be used, for example, in an M&D or Pandia digester.

As used herein, the term “surfactant” is a widely used contraction for “surface active agent” which is a generic term for materials that preferably absorb at interfaces as a result of the presence of both lyophylic and lyophobic structural units, the absorbtion generally resulting in the alteration of the surface or interfacial properties of the system. The term surfactant includes, but is not limited to oil in water emulsions. Surfactants impart desirable properties such as reducing bonding, improving absorbency or reducing friction. Surfactants are widely used in the pulp and paper industry for numerous enhancements. The classes of surfactants include anionic, cationic, nonionic, or ampholytic (amphoteric)/zwitterionic surface active materials. Examples of anionic surfactants include sodium stearate, sodium oleate, sodium dodecyl sulfate, sodium dodecyl benzene sulfonate, polyether sulfate, phosphate, polyether ester and sulfosuccinate. Examples of cationic surfactants include dodecylamine hydrochloride, hexadecyltrimethyl ammonium bromide, cetyltrimethyl-ammonium bromide, and cetylpyridinium bromide. One class of surfactant is cationic surfactants based on quaternary ammonium compounds containing fatty type groups. Examples of non-ionic surfactants include polyethylene oxides, sorbitan esters, polyoxyethylene sorbitan esters, and alkylaryl polyether alcohols. Examples of commercial surfactants are Presstige™ FC8160 (U.S. Pat. No. 4,861,429 and U.S. Pat. No. 4,895,62) and Dispel®73 (dipropylene glycol methyl ether), both non-ionic surfactants. Examples of ampholytic (amphoteric) or zwitterionic surfactant include imidazoline derivatives, betaines, sulfobetaines, and phosphatides.

In one embodiment the surfactant is applied to a wood furnish comprising pin chips and fines from a solution of from 10 to 2000 ppm. In another embodiment the surfactant is applied from a solution of from 100 to 1000 ppm and in yet another embodiment it is applied from a solution of from 500 to 750 ppm. The surfactant can also be applied to the entire wood furnish of over sized chips, thick chips, pin chips and fines. In this embodiment the surfactant is applied from a solution of from 0.01 to 2 weight percent in water. In another embodiment the surfactant is applied from a solution of from 0.1 to 1 weight percent in water and in yet another embodiment the surfactant is applied from a solution of from 0.5 to 0.75 weight percent in water.

Surfactant Use EXAMPLE 1

A wood furnish mixture of pin chips and fines from a Douglas fir blend of wood chips was obtained from gyratory screen rejects which had passed through a 3 mm screen. The feed mixture contained 19.5% pin chips and 80.5% fines as determined by SCAN-CM 40: 94. Before treatment the pin chip fraction was 51.4 percent and the fines fraction was 47.1 percent (control accepts). The mixture was sprayed with a solution of 500 ppm Presstige™ FC8160 and then passed through a second gyratory screen with a perforated deck to separate the pin chips from the fines fraction. The accepts fraction of the mixture increased from 51.4 to 61.2 percent pin chips and the fines fraction decreased from 47.1 to 36.9 percent fines as determined by SCAN-CM 40: 94. This demonstrates the effectiveness of adding the surfactant in separating the two fractions. When the fines fraction was further screened on the classifier the pin chip fraction was 1.1 percent and the fines fraction 98.9 percent. All values are on an oven dry basis. The data are presented in Table 1.

Drying

Wood chips normally have a solids content of about 48 to 50 percent. Pin chips can be separated from the fines fraction by removing or reducing the surface water content of the chips. They can also be separated by reducing the solids content of the wood and then separating the fines from the pin chips. As defined herein, “solids content” means the weight of wood oven dried at 100° C. overnight divided by the total weight of the wood. The resulting value is expressed as a percent. In one embodiment the wood chips are air dried to a solids content of at least 85 percent (15 percent moisture or less). In another embodiment the wood chips are air dried. Air dried wood is wood that has 11 percent or less moisture (89 percent or more solids). In another embodiment the wood chips are surface dried with blown heated air from about 90° C. to about 200° C.

EXAMPLE 2

A wood furnish mixture of pin chips and fines from a Douglas fir blend of wood chips was obtained from gyratory screen rejects which had passed through a 3 mm screen. The mixture contained 19.5% pin chips and 80.5% fines as determined by SCAN-CM 40: 94. Before treatment the pin chip fraction was 51.4 percent and the fines fraction was 47.1 percent (control accepts). The mixture was air dried and passed through a second gyratory screen with a perforated deck to separate the pin chips from the fines fraction. The accepts fraction of the mixture increased from 51.4 to 73.8 percent pin chips and the fines fraction decreased from 47.1 to 24.1 percent fines as determined by SCAN-CM 40: 94. The fines fraction of the rejects contained 1 percent pin chips and 99 percent fines as determined by SCAN-CM 40: 94. The results are presented in Table 1.

TABLE 1 Pin Chip Recovery Sample % Pin Chips % Fines Feed mixture 19.5 80.5 Control - Accepts 51.4 47.1 Controt - Rejects 1 99 Surfactant Treated - Accepts 61.2 36.9 Surfactant Treated - Rejects 1.1 98.9 Dried - Accepts 73.8 24.1 Dried - Rejects 1 99 

1. A method for recovering pin chips from a wood furnish comprising the steps of: providing a mixture comprising pin chips and fines; treating said pin chip and fines mixture with a surfactant to separate said fines from said pin chips; separating said pin chips from said fines mixture; wherein said separated pin chips are subsequently used for fiber recovery.
 2. The method of claim 1 wherein said surfactant is a non ionic surfactant.
 3. The method of claim 1 wherein said surfactant is anionic surfactant.
 4. The method of claim 1 wherein said surfactant is a cationic surfactant.
 5. The method of claim 1 wherein said surfactant is a ampholytic/zwitterionic surfactant.
 6. The method of claim 1 wherein the surfactant is applied from a solution of from 100 to 5000 ppm.
 7. The method of claim 1 wherein the surfactant is applied from a solution of from 500 to 1000 ppm.
 8. The method of claim 1 wherein the surfactant is applied from a solution of from 500 to 750 ppm.
 9. A method for recovering pin chips from a wood furnish comprising the steps of: providing a mixture comprising pin chips and fines; drying said pin chip and fines mixture to separate said fines from said pin chips; separating said pin chips from said fines mixture; wherein said separated pin chips are subsequently used for fiber recovery.
 10. The method of claim 9 wherein the surface of the pin chips is air dried.
 11. The method of claim 9 wherein said mixture of pin chips and fines are dried to a solids level of at least 85 percent.
 12. The method of claim 9 wherein said mixture of pin chips and fines are dried to a solids level of at least 89 percent. 